Concrete Masonry Units are an excellent building material for commercial and industrial building. Its flexibility and ease of installation attributes provide a cost-effective, durable and attractive end result. There is flexibility in regards to the design and construction.
Often no skilled labor or heavy machinery is required allowing for reduction of labor and equipment. Its strength and durable composition along with its green building advantages make CMU a strong building alternative to other building options.
Control of Wall Movement
All building materials that are used in general construction are subject to movement. When this movement occurs and is restrained, unsightly cracks may develop. The movement in masonry materials is generally due to changes in the moisture content and temperature. Normally, the net effect of this movement is shrinkage. The quantity of movement is associated with the type of masonry unit used, the quantity of reinforcing used, the shrinkage coefficient of the masonry units and the length to height ratio of the wall. There are three main changes that cause these stresses in concrete masonry:
- Physical change due to loading
- Thermal movement – Changes in temperature cause structures to expand or contract as the temperature increases or decreases. A change of 50 degrees F. will affect a 100-foot concrete wall with forces of about 410 PSI as much as ¼ inch. Since the tensile strength of the wall is generally less than that, cracking generally occurs.
- Changes in moisture content – Shrinkage due to moisture changes from uncontrolled means also cause changes in wall length. The amount of moisture content in the masonry units at the time it is placed in the wall determines the effects of shrinkage. The environment and weather conditions do and can change during construction.
Control joints are vertical joints, which provide continuous separation in the masonry to allow freedom of movement so as to relieve any build up of stress. These joints should be located where cracking (or stress) would be most likely to occur, such as in long straight walls, abrupt changes in wall thickness or heights, chases for pipes, at openings, columns or pilasters and any other points of potential excess tensile stress. They may also be used at wall intersections in main walls and partitions. Various types of control joints are shown in this section.
Factors that Affect Joint Spacing
Use of horizontal reinforcing helps to accommodate movement by increasing the tensile strength of the wall. Steel reinforcing may be added either by incorporating bond beams into the structure or by using joint reinforcing. Bond beams are also normally used to carry structural loads, such as over openings in a wall and the steel reinforcing is normally sized according to the magnitude of these loads. If bond beams are to be used as tensile reinforcing they should be reinforced with a minimum of two No. 4 bars; however, structural engineers should always be referred to when making these decisions. Reinforcing serves to increase the tensile strength of the structure. It may be spaced 8, 16 or 24 inches apart. The enclosed tables show the effect of joint spacing when joint reinforcing
Length To Height Ratio-
The length to height ratio for a masonry wall also affects the distance between control joints until the proper spacing is achieved. Care should be taken not to exceed recommended joint spacing for these ratios to ensure good performance.
In estimating the proper spacing of control joints, consideration is given to the height of the wall or vertical distance between major horizontal restraints, such as floors, roofs, etc. Usually this distance is equal to story height.
This illustration at the right shows the simple application of control joints as related to the height (h) and length (L) of a wall. The amount of horizontal joint reinforcement has a direct affect on the spacing control joints as related to L/h. Recommended control joint spacing not to exceed 25ft (7.62m) Ref. NCMA TEK 10-2B